Free 500 Rule & NPF Calculator
The maximum shutter time before stars visibly trail ā computed three ways: the classic 500 rule, the stricter 300 rule for high-res sensors, and the NPF rule, which accounts for your camera's actual pixel size and aperture.
Your setup
Max exposure times
Which rule should you use?
- NPF rule ā the accurate one. Formula: t = (35Ā·N + 30Ā·p) / f, where N is aperture, p is pixel size in Āµm, and f is the full-frame equivalent focal length. It accounts for both your sensor's pixel density and lens speed. Use this.
- 500 rule ā t = 500 / (focal Ć crop). Fine for older, lower-resolution sensors; visibly trails on 24 MP+ cameras at 100% view.
- 300 rule ā t = 300 / (focal Ć crop). A safer general choice for modern high-res sensors.
All three assume you're shooting away from the celestial poles; pointing near Polaris (or Sigma Octantis) lets you push times 2ā3Ć longer before trails appear.
Related tools & guides
Astro Pixel Scale Calculator
Is your sampling matched to your optics and seeing?
Open āDepth of Field for Astro
Focusing at infinity, the f-ratio trade-off, and stacking.
Read āExposure Equivalence
Balance ISO and shutter to hit the exact exposure.
Open āStar exposure FAQ
Why do stars trail at long exposures?
The Earth rotates ~15Ā°/hour. A longer shutter moves each star across more pixels; when the movement exceeds about one pixel (or the pixel-size tolerance in NPF), the star becomes a streak instead of a point.
Does the 500 rule still work on my 45 MP camera?
Poorly ā it was calibrated for 12ā16 MP sensors. At 45 MP the pixels are ~4.4 Āµm and trails are visible well before 500/f. The NPF rule (or 300/f) is the right choice for modern sensors.
What if I want star trails on purpose?
Then ignore all rules ā stack many long exposures (30 sā5 min each) and composite. The calculator's job is the opposite: keeping stars as points.